1. Field of the Invention
The present invention relates to an image forming apparatus having a pre-transfer lamp adapted for use in an office machine, such as an electro-photograph type printer, a copying machine, and a facsimile machine (FAX), and more particularly, to an image forming apparatus including a subsidiary transfer part having an optic fiber guide leading light emitted from a pre-transfer lamp onto a photosensitive element, such as a photosensitive drum, to expose the photosensitive element to light, thereby transferring a toner image efficiently onto a printing paper.
2. Description of the Related Art
Generally, an image forming apparatus which is applied to an office machine, such as a copying machine, a FAX, and a printer, comprises a laser scanning unit (LSU) forming a latent electrostatic image on a photosensitive element, such as a photosensitive drum, by scanning light fluxes, such as laser beams, thereon according to image signals, and an image forming part developing the latent electrostatic image on the photosensitive element using a developer composed of toner of given colors to form a toner image and transferring the toner image onto a printing paper to obtain a required image. The toner image transferred onto the printing paper is fixed thereon through a fusing part fusing (fixing) the toner image, and the fixed printing paper is then discharged outwardly from the office machine by a discharging part.
Lately, pre-transfer lamps, such as light-emitting diodes (LED), are disposed as a subsidiary transfer part to decrease or lower an electrical potential on a surface of the photosensitive drum, thereby enhancing a transfer efficiency in a transfer operation by exposing the photosensitive drum to light before the toner image is transferred onto the printing paper.
Referring now to FIG. 1, a conventional developing device 10 and its circumferential devices are schematically illustrated to form an image forming part of a conventional image forming apparatus, in which pre-transfer lamps are disposed.
The conventional image forming apparatus comprises a photosensitive drum 11 disposed to project partially and downwardly from the developing device 10, and a transfer roller 18 disposed to project partially toward a paper-transporting passage 20 defined by upper and lower frames 21, 22 of a main body of the image forming apparatus. The photosensitive drum 11 and the transfer roller 18 are arranged to contact each other with a given pressure when the developing device 10 is mounted on the main body.
At one side of the photosensitive drum 11, a charging roller 12 charging a surface of the photosensitive drum 11 to a high voltage is disposed to come in contact therewith, whereas at the other side of the photosensitive drum 11, a developing roller 13 supplying toner 15 onto the photosensitive drum 11 is disposed to rotate in a slightly spaced-apart relation therewith. Also, at one side of the developing roller 13, a toner-supplying roller 14 supplying the toner 15 from a toner container 16 of the developing device 10 to the developing roller 13 is arranged to come in contact with the developing roller 13 and to rotate in the same direction as the photosensitive drum 11.
Over the photosensitive drum 11 between the charging roller 12 and the developing roller 13, a laser scanning unit (LSU) 17 forming a latent electrostatic image on the surface of the photosensitive drum 11 charged with the high voltage by the charging roller 12 is disposed.
In the lower frame 21 of the main body forming the paper-transporting passage 20 below the photosensitive drum 11 of the developing device 10, a transfer roller 18 is disposed to engage the photosensitive drum 11, and at an upstream side of the transfer roller 18 in a paper-transporting direction, a printed circuit board 30 having a plurality of pre-transfer lamps 31 is disposed in the lower frame 21.
The pre-transfer lamps 31 are covered with a transparent cover 32, so that they can be protected. The pre-transfer lamps 31 function to expose the surface of the photosensitive drum 11 to light so as to lower an electrical potential thereof before a toner image is transferred onto a printing paper 50 by the transfer roller 18 later on, and thereby to increase the transfer efficiency.
However, the image forming apparatus described above has a structure in which the surface of the photosensitive drum 11 is exposed to the light passing through the printing paper 50, since the pre-transfer lamps 31 are disposed in the lower frame 21 below the paper-transporting passage 20. Therefore, according to a thickness of the printing paper 50, transmissivity of the light becomes different. As a result, a difference in an erasing capacity of the pre-transfer lamps 31 occurs. Accordingly, when the transmissivity of the light is decreased by the thickness of the printing paper 50, the transfer efficiency is lowered.
Also, in the conventional image forming apparatus, a plurality of the pre-transfer lamps 31, for example, 16 pre-transfer lamps, having a relatively low intensity of radiation should be longitudinally arranged along the photosensitive drum 11 since the pre-transfer lamps 31 are disposed to be close to the photosensitive drum 11. In this case, an uneven radiation of the pre-transfer lamps 31 results in a Ver-steak phenomenon, in which the photosensitive drum 11 is unevenly exposed to light due to luminosity declination of the respective pre-transfer lamps 31. Also, a fabrication cost may be increased since the pre-transfer lamps 31 are used and spaced-apart from each other.
To solve these problems, there has been proposed another image forming apparatus having a pre-transfer lamp and a light guide formed as a subsidiary transfer part not to interfere with a transported printing paper.
Referring to FIG. 2, a developing device 10′ and circumferential devices of the image forming apparatus include pre-transfer lamps 31′ and a light guide 40.
As illustrated in FIG. 2, the developing device 10′ of the image forming apparatus comprises the pre-transfer lamps 31′ disposed on a printed circuit board 30′ on an upper frame 22′ of a main body forming a paper-transporting passage 20′ of a printing paper 50′, and a light guide 40 having a wide width disposed between the pre-transfer lamps 31′ and a photosensitive drum 11′ to lead (guide) light emitted from the pre-transfer lamps 31′ onto a surface of the photosensitive drum 11′. The developing device 10′ includes a toner container containing toner 15′, a toner supply roller 14′, a developing roller 13′, and a toner layer regulating blade 13a. The image forming apparatus further includes a photosensitive drum 11′, a charging roller 12′, a lower frame 21′, and a toner container 16′, and laser scanning unit 17′, a transfer roller 18′.
Referring to FIG. 3, the light guide 40 formed of acryl material has an inclined surface 41 formed parallel with the surface of the photosensitive drum 11′ at an end thereof to radiate light in a direction vertical to the surface of the photosensitive drum 11′.
However, in the image forming apparatus described above, the problem causing a variation of the transmissivity of the light according to the thickness of the printing paper 50′ might be solved by using the light guide 40 having the wide width to lead (guide) light from the pre-transfer lamps 31′ onto the photosensitive drum 11′ above the printing paper 50′ and then to expose the surface of the photosensitive drum to light, but a new problem occurs in that a large space is required to dispose the light guide 40 having a bulky size between the pre-transfer lamps 31′ and the photosensitive drum 11′, thereby resulting in an increase in size of the image forming apparatus.
Also, in the image forming apparatus, there is a problem that a large number of the pre-transfer lamps 31′ should be still used to expose an overall surface of the photosensitive drum 11′ uniformly to the light.